[Rennes Templar]

“I’ve filled up a tub with hot water and slept in the bathroom for at least 2 hours at a time for warmth,”

How does that work?

[amorphous]

The hot water in the tub acts as a kind of heat battery. Don't worry about knowing when to drain and refill it. You'll wake up when it starts getting really cold. If the bathroom is next to the bed, you could probably sleep there.

Some let the water run continuously at a reduced rate, and let the tub's overflow outlet deal with the excess. Seems kind of wasteful, but you can get more sleep that way.